Abrasive grinding balls and method of manufacturing same



8 Claims. ci. 75-204) This invention relates to grinding balls such asare used in ball mills, and is a division of the co-pending applicationbearing the same title, in the names of the present inventors, and filedJanuary 13, 1958, Serial No. 708,357, now Patent No. 3,061,209. Ballmills are used in various industries for comminuting various materials.For instance, in the southwestern United States, in the cement andcopper mining industries, grinding balls are used in rotating ball millsin reducing the copper ore and other materials into finely dividedparticles.

In a pulverizing or grinding apparatus commonly known as a ball mill,various sizes of loose grinding members or elements are utilized togrind by means of a combination of a rolling, rubbing and poundingaction. These grinding elements are ordinarily referred to as balls,even though they are not truly spherical. Sometimes they are referred toas slugs. For this reason, the term ball as used in the specificationand claims is not used in its narrow sense of sphere, but in the sensein which it is used in the ball mill art, that is, to designate one of acollection of loose grinding elements, which are caused to roll andtumble about in a suitable container such as a drum or the like, in agrinding mill.

In order for a grinding ball to be an effective grinding medium, itrnusthave a certain combination of characteristics. It must be hard andabrasion resistant so that it will not erode faster than the materialwhich is being pulverized. At the same time, it must be tough enough sothat it will not fracture and split up upon use in the mill, as byimpact with the material which is being comminuted, or by impact withother balls or the mill lining, or by the crushing action of the load ongrinding balls at the bottom of the mill.

Our experience with grinding balls has shown that it would be verydesirable to have a grinding ball in which extremely hard particles areembedded in order to make the grinding ball more highly abrasive. Thereis a de mand for a grinding ball carrying finely divided abrasive suchas alumina, tungsten carbide, titanium carbide, silicon carbide, and thelike. Various attempts have been made to incorporate abrasive particlesinto grinding balls but these have not come into widespread use. Thereason why the incorporation of abrasive particles into a grinding ballpresents difiiculties is that generally speaking abrasive materials donot have the same density as iron or steel. Abrasive materials having aMohs hardness of 7 or above such as alumina, silicon carbide, andvarious borides and the like, are all too light to be incorporated intomolten iron or steel because they float to the surface before they canbe trapped and held in the freezing metal. Others, such as tungstencarbide, which would do the job, are too heavy and would sink to thebottom in a molten mixture before they could be trapped and held in thefreezing metal.

If powder metallurgy is employed, it is a simple matter to distributethe abrasive particles uniformly through the powdered iron and when thematerial is compacted and sintered, a uniform product is obtained.However, an abrasive ball produced by powder metallurgy will not bemetallurgically hard. The iron itself will be soft enough to deformunder impact and the ball will not 3,141,758 Patented July 21, 1964retain its original shape. Heat treatment of such a ball is difficultbecause of the fact that the abrasive particles have an insulatingeffect in that they do not readily conduct heat between the adjacentiron particles. If powdered iron is used in the powder metallurgytechnique, there is further disadvantage that the ball cannot behardened by heat treatment.

With the foregoing considerations in mind, it is an object of thepresent invention to overcome the difficulties heretofore encounteredand to produce an abrasive grinding ball which will not only haveparticles of abrasive embedded in a matrix of steel, but which will betough and hard so that it will not split or crack or deform when used ina ball mill.

It is another object of the invention to provide a grinding ball asabove outlined by powder metallurgy techniques, first forming acylindrical or other shaped compact, and then forging the sinteredcompact to a ball to further compact the particles, and then heattreating the forged ball to give it the required hardness.

These and other objects of the invention which will be pointed out inmore detail hereinafter or which will be apparent to one skilled in theart upon reading these specifications, we accomplish by that compositionand by that series of method steps of which we shall now describeexemplary embodiments.

A grinding ball having the desired qualities could be prepared by theaddition of abrasive material to the molten steel if the abrasive couldbe prevented from either settling out, or floating out, of the moltensteel. Similarly it may be possible to add to the molten steel certainalloying ingredients which, upon solidification of the steel, wouldproduce scattered inclusions of refractory compounds of an abrasivenature.

Briefly, in the preferred practice of the invention, We resort to powdermetallurgy and mix powdered iron or steel and finely comminuted abrasiveparticles in the desired portion, compact them into a compact, which,because the compacting is generally accomplished by means of a pistonand cylinder, will usually be cylindrical, and then sinter the compactat a temperature of about 2000 F. or higher, or in any event below themelting point of the powdered metal being used, having in mind theatmosphere being used. For example, a suitable atmosphere may beachieved by carrying out the sintering operation with the compactspacked in coke.

The compacted slug is then given a number of forging blows, having beenreheated to forging temperature if necessary. The forged ball is thenheat treated to secure the final physical properties desired. It will beunderstood by those skilled in the art that if powdered iron is used,steps must be taken at some stage of the process to render the iron heathardenable.

A typical abrasive for reasons of economy is alumina (A1 0 andpreferably it is finely comminuted to mesh or smaller. It will beunderstood that the more finely the abrasive is comminuted, the moredense it is possible to make the ultimate grinding ball; and naturallyalso the less alumina is used, the denser the ball can be made. Theupper limit of the amount of abrasive which may be added appears to beabout 30% of alumina by volume. Excellent results are achieved withalumina between 3% and 10% by volume. The iron or steel powder andabrasive powder are thoroughly and uniformly mixed and are compactedinto roughly cylindrical or other shape compacts.

It will be understood that, in order to end up with a heat treatableproduct, the ferrous matrix must contain some carbon. Thus if powderediron is used, carbon must be added either by mixing powdered carbon intothe compact if desired, or by sintering the iron compact undercarburizing conditions. If powdered steel (i.e.,

3 iron containing carbon) is used, decarburization must be prevented.This may be done by placing the compacts into crucibles packed withcoke, and sintering at a temperature of 2000 F. or higher. In this Way aslug is produced which is ready for forging.

The slugs are heated to forging temperature if necessary and forged insuitable dies to bring them to spherical shape. As an example, we haveforged the cylindrical slugs with three blows, the first blow achievinga substantially ball form and the second and third blows consolidatingthe flash formed in the first blow. The forging operation increases thedensity of the balls and renders them more readily heat hardenable.

The forged balls are then reheated, passed through sizing machines andheat treated. We have found it con venient to reheat the forged balls incoke prior to sizing and heat treating.

Balls manufactured as above described have an excellent appearance andcannot readily be distinguished from regular forged steel grinding ballsnow being produced, and as taught in the Hagenbuch and McCoy Patent No.2,182,805. Tests for hardness on balls produced as above show thatwhereas with no alumina the R hardness at .015 inch depth ranged from 64to 66, with alumina by volume the R ranged from 61 to 64, and with 30%alumina it ranged from 58 to 61. For commercially satisfactorygrindingballs the hardness at the surface should hold to a depth of inch fromthe surface, and drop only a little to the center of the ball. It willbe understood that hardness is necessary to prevent rapid wear anddamage to the ball surface in use.

With 3 to 10% alumina, the ball density is not lowered greatly and itshardness is not greatly affected while the advantages of abrasiveloading are achieved.

It will be understood that other abrasive materials may be used insteadof alumina, although tungsten carbide is relatively expensive and itwould be desirable to use not more than 3% by volume of tungsten carbideon account of the expense factor.

It will be clear that various modifications may be made withoutdeparting from the spirit of the invention. We do not intend to limitourselves except as set forth in the claims which follow.

Having now fully disclosed the invention, what is claimed as new anddesired to be secured by Letters Patent is:

1. The method of manufacturing a steel grinding element for use in ballmills and the like, comprising the steps of mixing powdered steel andless than 30% abrasive particles, compacting the mixed materials into amass and sintering the compacted mass at a temperature below the meltingpoint of the powdered metal in a carburizing atmosphere, forging theheated mass into final shape and heat treating the forged element toharden the matrix to at least R 58.

2. The method of manufacturing a steel grinding ball for use in ballmills and the like, comprising the steps of mixing powdered iron,powdered carbon, and less than 30% abrasive particles, compacting themixed materials into a cylindrical mass and sintering the compacted massat a temperature below the melting point of the powdered metal in acarburizing atmosphere, forging the heated mass into ball shape and heattreating the forged ball to harden the matrix to at least R 58.

'3. The method of claim 1, wherein the abrasive particles are present inan amount between 3% and 30% by volume.

4. The method of claim 3, wherein the abrasive particles are alumina.

5. The method of claim 4, wherein the alumina is in the form ofparticles of mesh or finer.

6. The method of claim 1, wherein the abrasive particles are present inan amount between 3% and 30% by volume, and said particles are tungstencarbide.

7. The method of claim 6, wherein the tungsten car bide is present inthe form of particles of 100 mesh or 8. The method of claim 1, whereinthe forged element is sized before heat treatment.

References Cited in the file of this patent UNITED STATES PATENTS1,794,300 Kelly Feb. 24, 1931 2,068,848 De Bats Jan. 26, 1937 2,404,598Sachse July 23, 1946 FOREIGN PATENTS 702,101 Great Britain Jan. 6, 1954712,082 Great Britain July 14, 1954

1. THE METHOD OF MANUFACTURING A STEEL GRINDING ELEMENT FOR USE IN BALLMILLS AND THE LIKE, COMPRISING THE STEPS OF MIXING POWDERED STEEL ANDLESS THAN 30% ABRASIVE PARTICLES, COMPACTING THE MIXED MATERIALS INTO AMASS AND SINTERING THE COMPACTED MASS AT A TEMPERATURE BELOW THE MELTINGPOINT OF THE POWDERED METAL IN A CARBURIZING ATMOSPHERE FORGING THEHEATED MASS INTO FINAL SHAPE AND HEAT TREATING THE FORGED ELEMENT TOHARDEN THE MATRIX TO AT LEAST RC58.